The present invention is directed to magnetorheological (MR) fluids suitable for use in controllable high compression vibration dampening devices. More specifically, the invention is directed to MR fluids that provide durability over long-term use in controllable high compression vibration dampening devices. The MR fluids of the present invention are comprised of mechanically hard magnetizable particles, a carrier fluid derived from a polyalphaolefin and a plasticizer, and a non-oligomeric thixotropic agent. The MR fluid formulations of the present invention have been found to uniquely provide long-term durability in magnetically controllable high compression vibration dampening devices.
Magnetorheological (MR) fluids are substances that exhibit the rather unique property of being able to reversibly change their apparent viscosity through the application of a magnetic field. For a MR fluid, the apparent viscosity and related flow characteristics of the fluid can be varied by controlling the applied magnetic field. Such fluids have wide application in vibration dampening devices such as, for example, shock absorbers, vibration dampers, force/torque transfer (clutch) devices, and the like, and especially in systems in which variable control of the applied dampening/force is desirable.
MR fluids are generally suspensions of finely divided magnetizable particles in a base carrier liquid. The particles are typically selected from iron, nickel, cobalt, and their magnetizable alloys. The base carrier liquid is generally a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid.
For commercial applications, the composition of MR fluids must have certain characteristics relating to durability, stability, viscosity, yield stress and volatility. With respect to durability, the fluid must be able to remain useful over a long period of time and must be minimally abrasive to the device in which it is housed. In MR fluids that contain metal particles, the natural selection has been toward those metal particles that are least abrasive, such as mechanically soft and compressible particles. Limited work has been done with mechanically hard particles due to their inherent abrasiveness and difficulty in creating stable fluid formulations. With respect to stability, the fluid formulation must be such that it limits particle settling. Thickeners and thixotropic agents have been used for this purpose, but it is important to select an agent that limits settling, while also limiting the apparent viscosity of the fluid in the xe2x80x9coff-statexe2x80x9d(i.e., when no magnetic field is applied). With respect to yield stress, the fluid formulation must be such that in the xe2x80x9con-statexe2x80x9d (i.e., when a magnetic field is applied) the fluid provides the desired dampening. With respect to volatility, it is desirable to select a fluid that has the lowest volatility without compromising on the viscosity of the fluid. Accordingly, the formulation of MR fluids is to a large degree dependent on the individual components selected.
The magnetizable particles used in prior art MR fluids have generally been selected from metal particles that are mechanically soft and easily compressible and which exhibit lower abrasion and wear to component surfaces. The magnetizable particle typically used in such prior art MR fluids has been reduced carbonyl iron that is known to be a mechanically soft and easily compressible metal particle having a nominal particle size of about 6-9 microns and a hardness of B50 on the Rockwell scale (generally equivalent to the hardness of brass). Examples of such MR fluids are illustrated, for example, in U.S. Pat. No. 4,992,190, and U.S. Pat. No. 5,167,850.
Typical grades of soft, reduced carbonyl iron available commercially are CL, CM, CS, CN, SP, SQ, SL, SD, SB, and SM grades manufactured by BASF, and the R-2430, R-2410, R-1510, R-1470, R-1430, R-1521, and R-2521 grades manufactured by ISP Technologies, Inc. These iron particles are magnetically soft, i.e., they magnetize under a magnetic field, but they lose their magnetism when the magnetic field is turned off. This soft magnetism allows chain formation and breakage, thus providing reversible off-state and on-state properties.
Various other metals and metal alloys have been disclosed for use by others, but the preferred magnetizable particle selected for use in MR fluids has remained reduced carbonyl iron. For example, U.S. Pat. No. 5,683,615, which relates to a MR fluid comprising magnetic-responsive particles with an average particle size distribution of about 1 to 100 microns, a carrier fluid, and at least one thiophosphorus or thiocarbamate, describes the use of high purity carbonyl iron as preferred for use in their fluid and select reduced carbonyl iron as the particle in their MR fluids. This selection of reduced carbonyl iron as the elected magnetic-responsive particle is similarly shown, for example, in U.S. Pat. No. 5,705,085, which relates to a MR fluid comprising magnetic-responsive particles with an average particle size distribution of about 1 to 100 microns, a carrier fluid, and at least one organomolybdenum; and also, in U.S. Pat. No. 5,906,767, which relates to a MR fluid comprising magnetic-responsive particles with an average particle size distribution of about 1 to 100 microns, a carrier fluid, and a phosphorus additive.
It is noted that U.S. Pat. No. 5,645,752, does disclose the use of a mechanically hard magnetizable particle, but it is distinguished over the present invention in that it does not provide a durable MR fluid formulation. U.S. Pat. No. 5,645,752, relates to a MR fluid comprising magnetic particles having a particle diameter ranging from about 1 to 500 microns, a carrier fluid and a thixotropic additive specifically limited to an oligomeric compound or a polymer-modified metal oxide.
The aforementioned MR fluids have proven to be useful in certain types of controllable vibration dampening devices in which the applied force is along a single axis, such as may be encountered with a rod and piston shock absorber that is mounted vertically (to a suspension system) and the applied force (or load) to the shock absorber is directed along the direction of the piston rod (i.e., vertically).
In many recent automotive applications, however, vibration dampening devices such as shock absorbers are no longer being solely mounted vertically in relation to the vehicle chassis and suspension system. Due to space limitations, and vehicle system requirements, it has become necessary in several applications for shock absorbers to be designed so that they can be mounted non-vertically. While the load forces may remain vertical in relation to the vehicle chassis, the applied forces to such non-vertically mounted shock absorbers are along multiple axes. This non-vertical force is referred to as the xe2x80x9cside load.xe2x80x9d
To accommodate the forces created by the side load, it has become necessary to redesign shock absorber systems to accommodate non-vertical applications. The primary efforts in this regard have been to redesign the shock tube and the piston, including hardening of the inner tube surface and plating of the surfaces of the piston head that come into contact with the inner tube surface.
MR fluids that contain soft, reduced carbonyl iron particles and use fumed silica as the thixotropic agent are known to thicken substantially during durability testing in dampers that have both a side load and a damping load. This thickening or paste formation causes the damping loads to increase sharply, thus compromising damper performance.
Several mechanisms, working individually or in combination, are believed to promote paste formation in MR fluids including the following:
(1) The action of the side load and the high rates of shear can cause severe deformation of the soft iron particles. Flattened and broken iron particles become adhered to each other when brought together under the influence of the magnetic field and then do not separate when the magnetic field is turned off. This causes agglomeration of the iron particles, resulting in fluid thickening.
(2) Fumed silica particles can mechanically bond to deformed soft iron particles due to the action of the side load. FIG. 1 shows an example of reduced carbonyl iron particles in an unused MR fluid; and FIG. 2 shows the particles in that fluid after 1 Million cycles of durability. The reduced iron particles after durability testing exhibit severe deformation and the fumed silica particles (seen as irregular fuzzy particles in FIG. 2) are mechanically attached to the iron particles. These fumed silica particles could not be removed from the iron particles using either solvent extraction or ultrasonic de-agglomeration techniques. It is believed that this mechanism can accelerate the agglomeration of the iron particles, resulting in quicker fluid thickening.
(3) When iron particles are deformed and broken, fresh pure iron is exposed. These fine particles of pure iron can act as catalysts and promote free radical polymerization of the carrier liquid. The deformation and breakage of such soft particles can accelerate polymerization of the carrier fluid molecules by catalysis and free radical mechanisms, thereby thickening the fluid.
While a durable MR shock absorber has been designed to withstand the side load on non-vertically mounted configurations, there is a need for correspondingly durable MR fluids. The present invention is directed to providing such durable MR fluids that address the desired yield stress properties for the device while exhibiting in the fluid long-term durability, sufficiently low viscosity and minimal particle settling, and to the device components minimal abrasion and wear.
The present invention is directed to durable MR fluid formulations comprising mechanically hard magnetizable particles, a carrier fluid derived from a polyalphaolefin and a plasticizer, and a non-oligomeric thixotropic agent.
It has been found that prior art formulations of MR fluids that are based on the use of mechanically soft magnetizable particles such as the reduced form of carbonyl iron are unable to maintain particle morphology and fluid consistency when subjected to long-term stress. FIGS. 1 and 2 show SEM photomicrographs of a MR fluid formulated according to fluids of the prior art using reduced carbonyl iron. In FIG. 1, the unused MR fluid shows that the particles have a spherical particle morphology. In FIG. 2, following 1 million cycles with a 100 Newton side load, however, the particle morphology has been completely disrupted. This is contrasted with the SEM photomicrographs of a MR fluid of the present invention, based on a formulation of the present invention using unreduced carbonyl iron, as shown in FIGS. 3 and 4. FIG. 3 shows the unused fluid; and FIG. 4 shows the fluid following 1 million cycles with a 100 Newton side load. As can be seen, the unreduced carbonyl iron particles in the MR fluid of FIGS. 3 and 4 substantially maintained their original spherical morphology and consistency.